The head of the U.S. Postal Services is pleading with Congress not to thwart his plan to cut Saturday mail as a way to save money.

But postal workers unions are criticizing the plan as illegal and financially questionable.

Postmaster General Patrick Donahoe says the plan announced last week is needed because the agency's budget is dire and getting worse.

In a hearing before a Senate committee Wednesday, lawmakers questioned the legality of the move and asked Donahoe for the legal opinion he based the announcement on.

Lawmakers also want a report from the comptroller general on how much money will be saved against how much will lost from decreased Saturday business.

Under the plan, scheduled to begin in August, package deliveries would continue six days a week.